The main switches, for example SW1, SW2, of a VF drive (variable frequency and variable voltage on the motor side) such as shown in FIG. 2 are controlled, first by the safety elements in a safety chain 6 shown in FIG. 3, second by inspection and recovery buttons and switches within a so called Inspection-Emergency Recovery (INS-ERO) box 8 (having inspection and recovery switches inside), and third by a control board 10, such as a printed circuit board with hard-wired discrete logic or with the drive control software on it. The meanings of the symbols shown within the board 10 are the same as identified in FIG. 1. Except for the control board 10 of FIG. 3, all elements are electrical safety devices according to the known European safety code EN81-1. It should be mentioned that the inspection mode or devices will include also a corresponding mode or device for a recovery run.
To fulfill the requirements setup in EN81-1 the chapter 14.1.2 "electric safety devices" has to be met, especially point 14.1.2.1.3., i.e., "Apart from exceptions permitted in this standard, no electric equipments shall be connected in parallel with an electric safety device."
Also to be met is point 14.1.2.1.5., i.e., "An output signal emanating from an electric safety device shall not be altered by an extraneous signal emanating from another electric device placed further down the same circuit, which would cause a dangerous condition to result."
The conventional method to fulfill the requirements in EN81-1 is done with the use of the additional relays U and D shown in FIG. 3, which is an expensive method. This method has also the disadvantage, in case of an inspection run: a hangup or stuck condition of the contacts of the U or D relay will keep the main switches SW1 and SW2 activated, although the inspection buttons are released. This error has to be detected by other means, such as an MSD (main switch disconnect) input in order to react upon the release of the inspection button by decelerating and stopping the drive.